PieCrust is a CMS, i.e. a Content Management System. It looks, by default, like a simple blog engine, but it can really handle any arbitrary set of content. You need however to define what that content is. This is the content model.
The default content model is what you get out of the box. It defines a simple blog site where you can have pages and posts.
To create more complex or customized websites, you can define another content model by specifying sources and routes in the site configuration:
Sources are how you tell PieCrust where to find your content. It defines what sub-directories contain what kind of pages, with what kind of naming convention, and what kind of metadata.
In theory, sources can also provide pages from other places than the file-system – like a network connection or a ZIP file or a database or whatever.
Some sources generate content on the fly, like the a source that lists all the tags or categories used by your blog posts.
Routes define how the content returned by the sources is exposed to your visitors. This defines both the URLs for that content, and the place on disk where it will be baked to.